1. Field of the Invention
This invention relates to tip caps for rotor blades and particularly to a new and improved replaceable tip cap having a unique structure for better cooling and longer life.
2. Description of the Prior Art
The efficiency of gas turbine engines can be increased by reducing the clearance between rotating and stationary parts. This is particularly important with respect to rotor blades, such as in a turbine, where tip losses, that is, air flowing around the end of, rather than streamwise across, the rotor blade, can occur because of too great a gap between the radially outer end of the rotor blade and a surrounding stationary shroud.
To reduce the size of the gap and thus reduce tip losses, the end of a rotor blade can be modified or a separate tip cap can be attached thereto. Such a blade end modification or a tip cap often comprises a recessed tip cavity between thin, radially extending blade edges, ribs or flanges, hereinafter cumulatively referred to as "blade edges". The rotor blade and its tip become hot during engine operation and must be cooled in some manner to provide strength and durability. Often, cooling of the rotor blade is accomplished by directing a fluid, such as pressurized air, through the hollow interior of the rotor blade. However, since the radially outer portions of the extending blade edges are located at a distance from the hollow portion of the internally cooled rotor blade, hot spots, that is, uncooled or inadequately cooled areas, can occur on the blade edges. In some rotor blades, such hot spots can be partially eliminated by including holes near the radially outer end of the rotor blade or in the base portion of the tip cap to direct air from the interior of the rotor blade to flow along or to impinge against the blade edges. However, even in such an arrangement, the cooling can be inadequate and hot spots can persist on the blade edges causing tip cracks and progressive oxidation to the point that the rotor blade must be prematurely removed from service for repair.
Another problem encountered with previous blade tips or tip caps is lack of wear resistance should a tip rub occur. The small cross-sectional area of the thin blade edges of the above-described blade tips or tip caps permits the blade edges to become highly heated and to wear relatively quickly. The resulting increase in the clearance between the rotor blade tip and the stationary shroud reduces engine efficiency. The engine efficiency loss is reflected in a higher turbine inlet temperature, and thus higher fuel consumption, required to achieve the same power output.
In view of the above-mentioned problems, therefore, an object of the present invention is to provide a replaceable tip cap for a rotor blade which includes an improved cooling arrangement therein to reduce tip cap overheating and thus increase tip cap life.
Another object of the present invention is to provide a replaceable tip cap with increased wear resistance which thereby reduces the rate of engine efficiency loss.